The present invention relates to a method of switching from a first coded video sequence to a second one, said video sequences having been encoded by different video encoders using regulation buffers, and to a corresponding switching device. This invention may be used for instance in MPEG1 or MPEG2 encoding/decoding systems.
Encoding systems such as those according to the MPEG2 standard produce data at variable rates (encoded pictures have variable sizes in bits) and require an output data buffer in order to feed the (constant bitrate) transmission channel. Similarly the corresponding decoders require an input data buffer to enable them to use data from said channel at the required variable rates. The transmitted encoded pictures will therefore not spend the same time in the decoder buffer. As a consequence, when switching from a first video sequence to a second one, the respective decoder buffer delays at the transition (or switching point) are not equal.
It is known indeed that, thanks to the abstract model of decoding VBV (Video Buffer Verifier) defined in the MPEG standard, it is possible to verify that an MPEG bitstream is decodable with reasonable buffering and delay requirements (expressed in the sequence header, in the fields "bitrate" and "buffer size"). This model of VBV is that of a receiving buffer for the coded bitstream and an associated instantaneous decoder so that all the data for a picture are instantaneously removed from said receiving buffer. Within the framework of this model, constraints on the bitstream (by way of the buffer occupancy) have been defined so that decoding can occur without buffer underflow or overflow. If said first and second sequences to be switched have been separately encoded, such a risk of overflow or underflow for the decoder buffer however exists.
In order to prevent such a risk, the switching operation may be performed in the decompressed domain where all the decoded pictures are again described with the same number of bits and last a same duration. Said pictures are then sent to a conventional mixer for switching, and recoded before transmission. Such a solution, described for example in the international patent application WO 97/08898, is however rather complex. Moreover, a decoding step followed by a re-encoding one damages the picture quality.